Lamp failure indicating apparatus

ABSTRACT

Apparatus for indicating inoperativeness of a lamp in any of several lamp arrays. Each lamp array includes one or more parallel connected lamps having predetermined electrical characteristics such that each operative lamp effects a certain voltage drop across the lamp array. A detector network monitors the voltage across each lamp array, which voltage is almost the same as the supply voltage, and generates a certain control signal when one of the lamps has become inoperative. An output amplifier that is responsive to the control signal effects energization of an indicator lamp to indicate that a lamp is inoperative when the certain control signal is generated. Several detector networks may be cascaded in parallel to monitor several lamp arrays and each detector network may be employed to monitor either one or two lamp arrays.

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[ LAMP FAILURE INDICATING APP ATlUS Primary Examiner-John W. CaldwellAssistant Examiner-William M. Wannisky [75] Inventor: James W. McNamee,Warren, Ohio Attorney-Jean L. Carpenter and Paul Fitzpatrick ABSCT [73]Assignee: General Motors Corporation,

Detroit, Mich.

22 Filed: Apr. 29, 1971 [21] Appl.No.: 138,602

Apparatus for indicating inoperativeness of a lamp in any of severallamp arrays. Each lamp array includes one or more parallel connectedlamps having predetermined electrical characteristics such that each340/248 operative lamp effects a certain voltage drop across the lamparray A detector network monitors the vow [58] Field ofSearch...................3l g(;gi2,sg5 1132948121511, g across each parray, which vohage is almost the same as the supply voltage, andgenerates a certain control signal when one of the lamps has become in-[56] References Cited UNITED STATES PATENTS 1/1969 Atkins...

operative. An output amplifier that is responsive to the control signaleffects energization of an indicator lamp to indicate that a lamp isinoperative when the 3,421,157 3,457,561 7,1969 Zeislerm" certaincontrol signal is generated. Several detector 3,629,799 12/1971 networksmay be cascaded in parallel to monitor 3,631,441 12/1971 Murphy...several lamp arrays and each detector network may be 3,593,264 7/1971Atkins..... employed to monitor either one or two lamp arrays. 3,171,0962/1965 Murray et al 3,535,585 10/1970 4 Claims, 1 Drawing Figure LAMPFAILURE INDICATING APPARATUS This invention relates to light systemsand, more particularly, to light systems in which failure of a lamp inany of several lamp arrays is indicated by an indicator device.

While the subject apparatus may be employed in conjunction with manydifferent types of equipment, it is particularly advantageous whenemployed in a motor vehicle which may have several lamp arrays ofsimilar electrical characteristics. For example, a motor vehicle mayemploy two headlamps in each of its front fenders for illuminating theroadway. The two headlamps referred to are often called high beams" andare connected in parallel to form a lamp array that is energized from acommon power supply through a single conductor. While there have beennumerous designs proposed for simultaneously monitoring two lamp arrayssuch as these to indicate when a lamp has failed in either of the lamparrays, these previous proposals have not met with much success for avariety of reasons.

Among these reasons are the fact that the prior art proposals generallyrequire that the apparatus monitoring the operativeness of the lampsconsume a considerable amount of standby power even while all of thelamps are operative. Another reason is the fact that many proposals haverequired that resistors be inserted in series with the lamp arrays beingmonitored to provide a significant, and measurable, voltage drop inseries with each of the lamp arrays and the power source. This proposalhas similarly not received widespread acceptance as it both consumes aconsiderable amount of power in the added resistors and reducesefficient operation of the lamps.

Another disadvantage common to many prior proposals for indicating thepresence of an inoperative lamp is the fact that each lamp array hasoften been monitored by a unique detector circuit which has not beenreadily adaptable for use in a system requiring additional detectorcircuits, or channels, to monitor the operativeness of other lamp arraysso that a common indicator device for indicating a failure inany of thelamp arrays being monitored could be used. Similarly, where a detectorhas been proposed as suitable for monitoring two or more lamp arrays, ithas often been unsatisfactory in monitoring the operativeness of asingle lamp. Both of the last mentioned disadvantages of the designsprevalent in'the prior art naturally result in considerable economicdisadvantage in applications where it is most advantageous to employ asingle detector unit that is readily adaptable in conjunction withsimilar detector units to monitor the operativeness of any number oflamp arrays.

It is therefore an object of this invention to provide a light systemwhich includes two lamp arrays and a detector network for indicatingwhen alamp has failed in either of the lamp arrays, the detector networkdrawing a minimum of power by monitoring a voltage substantially thesame as the voltage applied by the power source.

It is also an object of this invention to provide lamp failureindicating apparatus for indicating failure of a lamp, the apparatusbeing operable only when the lamps are energized and designed in modularform whereby several modular channels may be cascaded in parallel tomonitor a considerable number of lamp arrays while employing but asingle indicator device for 2 indicating failure of a lamp in any of thelamp arrays being monitored.

It'is an additional object of this invention to provide lamp failureindicating apparatus for indicating failure of a lamp in either of twolamp arrays across which there appears a voltage drop substantiallyequal to the voltage of an applied power source and for monitoring but asingle lamp, rather than all of the lamps in two lamp arrays, with aminimum of adaptation.

The foregoing and other objects and advantages of the subject inventionwill become apparent from the following description and accompanyingdrawing, in which is presented a schematic diagram of a light systemembodying the principles of the subject invention.

As mentioned previously, the subject apparatus is particularly usefulwhen employed in a vehicle. Accordingly, in the accompanying drawing alight system 10 is illustrated in which a suitable power source, such asa vehicle battery 12, energizes several lamps 14 through 17 through anormally open light switch 18 and first and second resistors 20 and 22that are each connected in series with two of the lamps 14 through 17.The lamps 14 through 17 are thus connected to form first and second lamparrays 24 and 26 that may each be mounted on one of the front fenders ofthe vehicle.

The voltages across each of the lamp arrays 24 and 26, which voltagesappear at first and second junctions 28 and 30, are monitored by a firstdetector network 32 that includes a differential comparator 34, acurrent source network 36, and a full wave detector 38 that generates acertain control signal when one of the lamps 14 through 17 isinoperative. An output amplifier 40 monitors the control signal andeffects energization of a lamp 42 when the certain control signal isgenerated.

The light system 10 also includes a second detector network 44 that issimilar to the first detector network 32 but which monitors theoperativeness of yet another lamp 46. By way of example, and withoutlimitation, the lamp 46 may be what is commonly termed a running lamp onthe vehicle and is energized through an ignition switch 48 and aresistor 50 whenever the vehicle is in operation. Should the runninglamp 46 become inoperative the second detector network 44 generates thecertain control signal and effects energization of the indicator lamp 42through the output amplifier 40.

The light system 10 will now be explained in detail.

The resistors 20 and 22 are selected to be of substantially the sameresistance. Similarly, the lamps 14 through 17 are each selected to havea certain resistance that is very much larger than the resistance of theresistors 20 and 22 so that the current through each of the resistors 20and 22 is directly proportional to the number of lamps drawing thecurrent. For example, if the battery 12 is selected to supply power at acontinuous potential of 12 volts and the resistance of each of the lamps14 through 17 is 599 times greater than the resistance of each of theresistors 20 and 22, each of the lamps 14 through 17 will draw a certaincurrent through the respective resistors 20 and 22. In actual practicethe resistors 20 and 22 have been determined by selecting the wiresconnecting the lamp arrays 24 and 26 to the battery 12 to have apredetermined low resistance rather than by selection of discreteresistor elements.

Accordingly, if it is assumed that current drawn by the first detectornetwork 32 is negligible when compared with the current drawn by thelamps 14 through 17, which assumption proves valid in practice, it thusfollows that a certain voltage drop is created across each of theresistors and 22 by the current drawn by each of the lamps 14 through17. In the example just described, the current through each of the lamps14 through 17 effects a 20 millivolt drop across the resistor in serieswith the lamp. It thus follows that in the illustrated embodiment, thevoltage at each of the junctions 28 and is 11.96 volts when all of thelamps 14 through 17 are operative. Should one of the lamps in either ofthe lamp arrays 24 or 26 become inoperative it will cease to drawcurrent and the voltage at the corresponding junction 28 or 30 will beincreased to l 1.98 volts. Accordingly, the first detector network 32 isdesigned to monitor the voltage at the junctions 28 and 30 and togenerate the certain control signal when the monitored voltages areunequal by 20 millivolts.

The junctions 28 and 30 are connected to the respective anodes of firstand second diodes 52 and 54 in the differential comparator 34. Thecathodes of the diodes S2 and 54 are connected to the base terminals offirst and second PNP transistors 56 and 58 having their respectiveemitter terminals connected to the anodes of the respective second andfirst diodes 54 and 52. The collector terminals of the first and secondtransistors 56 and 58 are connected to a point of reference potential,which is ground, through first and second bias resistors 60 and 62 and adiode 64 that protects the first and second detector networks 32 and 44from negligent reversal of the battery 12 polarity.

The current through the first and second diodes 52 and 54 is held at asubstantially constant level by the current source network 36. Thecurrent source network 36 includes a pair of resistors 66 and 68 and apair of diodes 70 and 72 connected between the second junction 30 andthe anode of the polarity reversal diode 64. A Zener diode 74 isconnected between the anode of the polarity reversal diode 64 and thejunction 76 between the resistors 66 and 68 so as to maintain thejunction 76 at a constant voltage.

The current source network 36 also includes a pair of NPN transistors 78and 80 and resistors 82 and 84 that are connected between the emittersof the transistors 78 and 80 and the anode of the polarity reversaldiode 64. The collectors of the transistors 78 and 80 are connected tothe cathodes of the respective first and second diodes 52 and 54 fordrawing a predetermined current through the first and second diodes 52and 54. The predetermined current level in a preferred design is on theorder of 200 microamps and hence is so minute as not to affect thevoltage at the junctions 28 and 30. The conductivity of the transistors78 and 80 is established by the voltage across the diodes 70 and 72,which voltage appears at a junction 86 that is connected to the bases ofthe transistors 78 and 80. Since the voltage across the diodes 70 and 72will vary with temperature changes the diodes 70 and 72 providetemperature compensation for the transistors 56 and 58 by controllingthe conductivity of the transistors 78 and 80, as will subsequentlybecome apparent.

When the first detector 32 is in operation the current source network 36establishes the predetermined current through the first and seconddiodes 52 and 54. The first and second diodes 52 and 54 are thus held ina conductive condition in which the voltage across each of the diodes 52and 54 remains substantially constant.

By selecting the diodes 52 and 54 to be matched so as to havesubstantially the same electrical characteristics the voltages which areapplied to the emitter, or input, terminals of the transistors 56 and 58are equal and the voltages applied to the bases of the transistors 56and 58 are also equal. The voltage drop across the first and seconddiodes 52 and 54, which may be on the order of 0.7 volts, thus maintainsthe first and second transistors 56 and 58 in a state of continuousconduction as class A operated amplifiers and their collector, oroutput, terminals conduct current through the bias resistors and 62. Byfurther matching the diodes 52 and 54 to have the same characteristicsas the emitter-base diodes of the transistors 56 and 58 the currentthrough the transistors 56 and 58 may be made substantially the same ascurrent through the diodes 52 and 54. This establishes the operatingpoints of the transistors 56 and 58 at the same level of conductivity.

The currents through the bias resistors 60 and 62 generate first andsecond voltage signals which are monitored by the full wave detector 38,which includes third and fourth PNP transistors 88 and 90. The bases ofthe respective transistors 88 and 90 are connected to the collectorterminals of the respective transistors 56 and 58 and the emitterterminals of the transistors 88 and 90 are connected to the collectorterminals of the respective transistors 58 and 56. The same potential isthus applied to the base and emitter terminals of the third and fourthtransistors 88 and 90 so long as the voltages on the collector terminalsof the first and second transistors 56 and 58 are equal. The third andfourth transistors 88 and 90 are thus held nonconductive and theircollector terminals, which are connected together, provide a low voltagecontrol signal so long as the lamps 14 through 17 are operative. Thecontrol signal is coupled by an isolation diode 92 to the outputamplifier 40 to control the energization of the indicator lamp 42, whichis connected to the battery 12 through a lead 94.

While the output amplifier 40 in the illustrated embodiment is a lampdriver circuit for controlling the energization of the indicator lamp42, persons versed in the art will appreciate that the output amplifier40 may instead be employed to control the energization of various otherindicator devices. In the illustrated embodiment, the output amplifier40 includes several cascaded NPN transistors 96 through 98 in what isknown as a Darlington configuration, a capacitor 99 for protecting thetransistors 96 through 98 from transients, an input resistor 100 throughwhich the control signal is transmitted from the isolation diode 92 tothe transistors 96 through 98, and a gain control network 102 whichconsists of a variable resistor 104 and several series connected diodes106 through 108. The diodes 106 through 108 are selected to match theemitter-base diodes of the transistors 96 through 98 so as to providetemperature compensation within the output amplifier 40.

The operation of the apparatus just described may be summarized asfollows. When all of the lamps 14 through 17 are energized, the voltagesat the junctions 28 and 30 are equal. The first and second transistors56 and 58, which have their electrical characteristics matched, are thusheld in a class A operation conductive condition and their output, orcollector, terminals are at the same voltage. The third and fourthtransistors 88 and 90 are held in a nonconductive condition by thesevoltage signals and the collectors of the third and fourth transistors88 and 90 provide a low voltage control signal to the amplifier 40. Thecontrol signal thus provided is insufficient to forward bias theemitter-base junctions of the transistors 96 through 98 in the outputamplifier so the transistors 96 through 98 are nonconductive and theindicator device 42 remains deenergized so long as all of the lamps 14through 17 are operative.

Should one of the lamps 14 through 17 become inoperative, such as thelamp 14, the voltage across the lamp array containing the inoperativelamp is increased from approximately 11.96 volts to 11.98 volts. In thisexample, the voltage increase takes place at junction 28 and causes thesecond transistor 58 to become more conductive by increasing its emittervoltage while its base voltage is unchanged. The increase in voltage atthe junction 28 effects a corresponding decrease in conduction of thefirst transistor 56 since the potential applied to its base by the firstdiode 52 increases from approximately 11.26 to 11.28 volts while thepotential applied to its emitter by the second junction 30 remains at11.96 volts. The resultant increase in current through the secondtransistor 58 and decrease in current through the first transistor 56effect a corresponding voltage increase across the second bias resistor62 and decrease in voltage across the first bias resistor 60.

Since the collector of the second transistor 58 is at a higher potentialthan the collector of the first transistor 56, the third transistor 88is biased to a conductive condition while the fourth transistor 90 ismaintained nonconductive. Once the transistor 88 has begun to conductthe voltage on its collector attains a certain predetermined level andthis certain control signal is suffrcient when coupled through theisolation diode 92 and the input resistor 100 to bias the transistors 96through 98 in the output amplifier 40 to a conductive condition, whicheffects energization of the indicator lamp 42 to indicate that one ofthe lamps 14 through 17 has failed.

While the indicator lamp 42 does not indicate which of the lamps 14through 17 have failed, it serves to advise the vehicle operator thatone of the lamps of the vehicle is inoperative so that he may visuallyascertain which of the lamps has failed. Since the indicator lamp 42 maybe energized whenever the certain control signal is applied to the inputresistor 100 any number of detector networks similar to the firstdetector network 32 may be combined to provide the certain controlsignal when various other conditions have occurred within the vehicle.

This adaptability of the first detector network 32 has been illustratedin the drawing by provision of the previously recited second detectornetwork 44, which monitors the operation of the running lamp 46. Thesecond detector network 44 is identical in configuration to that of thefirst detector network 32 except for the deletion of the fourthtransistor 90, which is unnecessary in the second detector network 44,and the employment of different resistance values for the first andsecond bias resistors 60 and 62. Accordingly, circuit elements in thesecond detector network 44 are noted with the same identificationnumerals as employed for corresponding circuit elements in the firstdetector network 32 followed by a prime notation.

The second detector network 44 is connected to monitor the voltage ateach side of the resistor 50. Accordingly, the voltage applied to theanode of the secllu and diode 54' is substantially 12 volts when thebattery 12 is a 12 volt supply and the voltage at the anode of the firstdiode 52' is approximately 11.98 volts. This voltage ofiset tends tobias the first transistor 56' more conductive than the second transistor58'. For this reason the biasing resistors 60' and 62' are adjusted sothe collector voltages of the first and second transistors 56' and 58are equal when the lamp is operative. Accordingly, the base and emittervoltages of the third transistor 88 are equal and the control signal onits collector is at a low potential so as to preclude the seconddetector network 44 from efiecting energization of the indicator lamp 42when the lamp 46 is operative. It should be noted that the biasresistors 60 and 62 in the first detector network 32 could similarly beadjusted to compensate for a voltage offset caused by one of the lamparrays 24 or 26 containing more lamps than the other lamp array 24 or26.

Should the lamp 445 become inoperative it will cease to draw current andsubstantially the entire potential of the battery 12 will be applied tothe anode of the first diode 52' as well as to the anode of the seconddiode 54'. This effects an increase in the voltage applied to the baseof the first transistor 56' so as to decrease its conductivity andeffect an increase in the conductivity of the second transistor 58'.Accordingly, the voltage on the base of the transistor is decreasedwhile its emitter voltage is increased. This turns on the transistor 88'and causes the generation of the certain control signal on itscollector. The certain control signal is coupled from the collector ofthe third transistor 88 through a second isolation diode to the outputamplifier 40 where it effects energization of the indicator lamp 42 inthe fashion previously explained. The single indicator lamp 42 may thusbe employed to indicate that one of any number of lamps has failed, thenumber only being limited by the number of detector networks employed.

Persons versed in the art will appreciate that various modification ofthis invention may be made without departing from its spirit.

What is claimed is:

1. In a light system having a first lamp array connected in series witha power source and a first resistor and a second lamp array connected inseries with the power source and a second resistor, each lamp arraycomprising a certain number of parallel connected lamps having similarimpedances,.the ratio of the impedance of thefirst lamp array to theimpedance of the first resistor being the same as the ratio of theimpedance' of the second lamp array to the impedance of the secondresistor, apparatus for indicating when a lamp in either of the lamparrays has failed comprising, in combination, a differential comparatorfor comparing the voltage across each of the lamp arrays and generatingfirst and second signals having relative magnitudes related to thedifierence between the voltages compared, the first and second signalsbeing equal when the monitored voltages are equal and unequal when themonitored voltages are unequal, a full wave detector that is responsiveto the first and second signals for generating a certain control signalonly when the first and second signals are unequal, the control signalbeing other than the certain control signal when the first and secondsignals are equal, an indicator device for indicating that one of thelamps has failed, and an output amplifier that is responsive to thecontrol signal for effecting an indication by the indicator device thata lamp has failed whenever the full wave detector generates the certaincontrol signal.

2. In a light system having a first lamp array connected inseries with apower source and a first resistor and a second lamp array connected inseries with the power source and a second resistor, each lamp arraycomprising a certain number of parallel connected lamps having similarimpedances, the ratio of the impedance of the first lamp array to theimpedance of the first resistor being the same as the ratio of theimpedance of the second lamp array to the impedance of the secondresistor, apparatus for indicating when a lamp in either of the lamparrays has failed comprising, in combination, a differential comparatorcomprising first and second matched diodes having their respectiveanodes connected to sample the voltage across the respective first andsecond lamp arrays, first and second matched amplifiers each having acontrol terminal and input and output terminals between which acontrolled current path is defined, the control terminals of therespective first and second amplifiers being connected to the cathodesof the respective first and second diodes and the input terminals of therespective first and second amplifiers being connected to the anodes ofthe respective second and first diodes, and first and second biasingimpedances connected in series with the output terminals of therespective amplifiers; means for providing a predetermined load inseries with each of the diodes whereby a predetermined potentialdiflerence is established across each of the diodes, the biasingimpedances being selected to effect operation of the amplifiers so as toestablish equal potentials on their output terminals when all of thelamps are operative and unequal potentials on their output terminalswhen a lamp has failed; detector means responsive to the potentials oneach of the output terminals for generating a certain output signal whensaid potentials are unequal; an indicator device for indicating failureof a lamp in either of the lamp arrays; and means responsive to thedetector means for effecting an indication by the indicator device thata lamp has failed when the detector means generates the certain outputsignal.

3. A light system comprising, in combination, first and second lamparrays, each lamp array including at least one grounded lamp; a powersource having a grounded terminal; first and second resistors connectingthe respective first and second lamp arrays in series with the powersource so as to connect each lamp in each lamp array in parallel withthe other lamps in the same lamp array, the ratio of the impedance ofthe first lamp array to the impedance of the first resistor being thesame as the ratio of the impedance of the second lamp array to theimpedance of the second resistor, the resistances of the lamps in eachlamp array being equal and very much greater than the resistorresistances whereby the current through each lamp effects apredetermined voltage drop across the resistor in series therewith;first and second diodes having their respective anodes connected to therespective junctions between the first resistor and the first lamp arrayand the second resistor and the second lamp array for partaking of thevoltage across each of the lamp arrays; first and second controlledamplifiers each having control, input, and output terminals, the controlterminals of the respective first and second amplifiers being connectedto the cathodes of the respective first and second diodes and the inputterminals of the respective first and second amplifiers being connectedto the anodes of the respective second and first diodes; first andsecond biasing impedances connected in series with the output terminalsof the respective first and second amplifiers whereby the voltages onthe output terminals are responsive to the conductivity of therespective amplifiers; means for drawing a predetermined current throughthe diodes so as to effect a predetermined voltage drop across each ofthe diodes of sufficient magnitude to render the amplifiers partiallyconductive, the magnitude of the biasing impedances being selected whenall of the lamps are operative so that each of the amplifier outputterminals are at a certain potential whereby failure of one of the lampseffects a change in the conductivity of the amplifiers and acorresponding difference in the potential on the output terminals of theamplifiers; a detector for generating a certain control signal when theoutput terminals are at difierent potentials, the detector includingthird and fourth controlled amplifiers each having control, input, andoutput terminals, the control terminals of the third and fourthamplifiers partaking of the potentials on the output terminals of therespective first and second amplifiers and the input terminals of thethird and fourth amplifiers partaking of the potentials on the outputterminals of the respective second and first amplifiers, the outputterminals of the third and fourth amplifiers being connected so as toprovide a common terminal at which the control signal is generated,equal potentials on the output terminals of the first and secondamplifiers biasing the third and fourth amplifiers to a first state ofconduction in which they generate a first control signal on the commonterminal and unequal potentials on the output terminals of the first andsecond amplifiers biasing the third and fourth amplifiers to a differentconductive state in which they generate the certain control signal onthe common terminal; an indicator device for indicating failure of alamp; and means responsive to the certain control signal for effectingan indication by the indicator device that a lamp has failed when thedetector generates the certain control signal.

4. In a light system having a grounded lamp connected in series with agrounded power source and a resistor whereby current through the lampefiects a predetermined voltage drop across the resistor, apparatus forindicating failure of the lamp comprising, in combination, adifferential comparator for monitoring the voltage across the resistorand for generating first and second signals indicative of theoperativeness of the lamp, the differential comparator including firstand second matched diodes having their anodes connected to therespective terminals of the resistor, the anode of the first diode beingconnected to the resistor terminal that is connected to the lamp and theanode of the second diode being connected to the other resistorterminal, first and second controlled amplifiers each having control,input, and output terminals, the confiol terminals of the respectivefirst and second amplifiers being connected to the cathodes of therespective first and second diodes and the input terminals of therespective first and second amplifiers being connected to the anodes ofthe respective second and first diodes, and first and second impedancesconnecting the respective output terminals of the respective first andsecond amplifiers to a point of reference potential whereby the firstand second signals are generated on the output terminals of therespective first and second amplifiers and an output terminal on whichis generated the control signal; an indicator device for indicating thatthe lamp has failed; and an output amplifier that is responsive to thecontrol signal and effective when the certain control signal isgenerated to cause the indicator device to indicate that the lamp hasfailed. I I0! 4 t t

1. In a light system having a first lamp array connected in series witha power source and a first resistor and a second lamp array connected inseries with the power source and a second resistor, each lamp arraycomprising a certain number of parallel connected lamps having similarimpedances, the ratio of the impedance of the first lamp array to theimpedance of the first resistor being the same as the ratio of theimpedance of the second lamp array to the impedance of the secondresistor, apparatus for indicating when a lamp in either of the lamparrays has failed comprising, in combination, a differential comparatorfor comparing the voltage across each of the lamp arrays and generatingfirst and second signals having relative magnitudes related to thedifference between the voltages compared, the first and second signalsbeing equal when the monitored voltages are equal and unequal when themonitored voltages are unequal, a full wave detector that is responsiveto the first and second signals for generating a certain control signalonly when the first and second signals are unequal, the control signalbeing other than the certain control signal when the first and secondsignals are equal, an indicator device for indicating that one of thelamps has failed, and an output amplifier that is responsive to thecontrol signal for effecting an indication by the indicator device thata lamp has failed whenever the full wave detector generates the certaincontrol signal.
 2. In a light system having a first lamp array connectedin series with a power source and a first resistor and a second lamparray connected in series with the power source and a second resistor,each lamp array comprising a certain number of parallel connected lampshaving similar impedances, the ratio of the impedance of the first lamparray to the impedance of the first resistor being the same as the ratioof the impedance of the second lamp array to the impedance of the secondresistor, apparatus for indicating when a lamp in either of the lamparrays has failed comprising, in combination, a differential comparatorcomprising first and second matched diodes having their respectiveanodes connected to sample the voltage across the respective first andsecond lamp arrays, first and second matched amplifiers each having acontrol terminal and input and output terminals between which acontrolled current path is defined, the control terminals of therespective first and second amplifiers being connected to the cathodesof the respective first and second diodes and the input terminals of therespective first anD second amplifiers being connected to the anodes ofthe respective second and first diodes, and first and second biasingimpedances connected in series with the output terminals of therespective amplifiers; means for providing a predetermined load inseries with each of the diodes whereby a predetermined potentialdifference is established across each of the diodes, the biasingimpedances being selected to effect operation of the amplifiers so as toestablish equal potentials on their output terminals when all of thelamps are operative and unequal potentials on their output terminalswhen a lamp has failed; detector means responsive to the potentials oneach of the output terminals for generating a certain output signal whensaid potentials are unequal; an indicator device for indicating failureof a lamp in either of the lamp arrays; and means responsive to thedetector means for effecting an indication by the indicator device thata lamp has failed when the detector means generates the certain outputsignal.
 3. A light system comprising, in combination, first and secondlamp arrays, each lamp array including at least one grounded lamp; apower source having a grounded terminal; first and second resistorsconnecting the respective first and second lamp arrays in series withthe power source so as to connect each lamp in each lamp array inparallel with the other lamps in the same lamp array, the ratio of theimpedance of the first lamp array to the impedance of the first resistorbeing the same as the ratio of the impedance of the second lamp array tothe impedance of the second resistor, the resistances of the lamps ineach lamp array being equal and very much greater than the resistorresistances whereby the current through each lamp effects apredetermined voltage drop across the resistor in series therewith;first and second diodes having their respective anodes connected to therespective junctions between the first resistor and the first lamp arrayand the second resistor and the second lamp array for partaking of thevoltage across each of the lamp arrays; first and second controlledamplifiers each having control, input, and output terminals, the controlterminals of the respective first and second amplifiers being connectedto the cathodes of the respective first and second diodes and the inputterminals of the respective first and second amplifiers being connectedto the anodes of the respective second and first diodes; first andsecond biasing impedances connected in series with the output terminalsof the respective first and second amplifiers whereby the voltages onthe output terminals are responsive to the conductivity of therespective amplifiers; means for drawing a predetermined current throughthe diodes so as to effect a predetermined voltage drop across each ofthe diodes of sufficient magnitude to render the amplifiers partiallyconductive, the magnitude of the biasing impedances being selected whenall of the lamps are operative so that each of the amplifier outputterminals are at a certain potential whereby failure of one of the lampseffects a change in the conductivity of the amplifiers and acorresponding difference in the potential on the output terminals of theamplifiers; a detector for generating a certain control signal when theoutput terminals are at different potentials, the detector includingthird and fourth controlled amplifiers each having control, input, andoutput terminals, the control terminals of the third and fourthamplifiers partaking of the potentials on the output terminals of therespective first and second amplifiers and the input terminals of thethird and fourth amplifiers partaking of the potentials on the outputterminals of the respective second and first amplifiers, the outputterminals of the third and fourth amplifiers being connected so as toprovide a common terminal at which the control signal is generated,equal potentials on the output terminals of the first and secondamplifiers biasing the third and foUrth amplifiers to a first state ofconduction in which they generate a first control signal on the commonterminal and unequal potentials on the output terminals of the first andsecond amplifiers biasing the third and fourth amplifiers to a differentconductive state in which they generate the certain control signal onthe common terminal; an indicator device for indicating failure of alamp; and means responsive to the certain control signal for effectingan indication by the indicator device that a lamp has failed when thedetector generates the certain control signal.
 4. In a light systemhaving a grounded lamp connected in series with a grounded power sourceand a resistor whereby current through the lamp effects a predeterminedvoltage drop across the resistor, apparatus for indicating failure ofthe lamp comprising, in combination, a differential comparator formonitoring the voltage across the resistor and for generating first andsecond signals indicative of the operativeness of the lamp, thedifferential comparator including first and second matched diodes havingtheir anodes connected to the respective terminals of the resistor, theanode of the first diode being connected to the resistor terminal thatis connected to the lamp and the anode of the second diode beingconnected to the other resistor terminal, first and second controlledamplifiers each having control, input, and output terminals, the controlterminals of the respective first and second amplifiers being connectedto the cathodes of the respective first and second diodes and the inputterminals of the respective first and second amplifiers being connectedto the anodes of the respective second and first diodes, and first andsecond impedances connecting the respective output terminals of therespective first and second amplifiers to a point of reference potentialwhereby the first and second signals are generated on the outputterminals of the respective first and second amplifiers, the first andsecond signals being equal when the lamp is operative and unequal whenthe lamp is inoperative; a detector that is responsive to the first andsecond signals and is effective to generate a certain control signalwhen the first and second signals are unequal, the detector including athird controlled amplifier having control and input terminals connectedto the output terminals of the respective first and second amplifiersand an output terminal on which is generated the control signal; anindicator device for indicating that the lamp has failed; and an outputamplifier that is responsive to the control signal and effective whenthe certain control signal is generated to cause the indicator device toindicate that the lamp has failed.